Various regulating methods of this type are known from the field of process control. They can be classified as either analog or digital. Although analog methods respond practically without delay, they are not very flexible. Digital methods are more flexible but, due to their inherent sampling times, can respond only subsequent to a certain delay. Such delays are a great drawback in the operation of turbines because they demand an increase in the margins of safety in relation to dangerous or impermissible operating states, restricting the exploitable operating range of the turbine. The restrictions in response time can to some extent be avoided by employing a digital method of control and regulation that uses a microprocessor system programmed in an assembler language. One drawback is that flexibility is again powerfully decreased and that programming in assembler language is very expensive and demands highly specialized skills.
German Patent 3 105 376 discloses a regulating method that represents a combination of analog and digital approaches. The basic regulation is accomplished with an analog regulator with output signals that are corrected by a supplementary regulator. The corrections optimize the regulating characteristic. The analog regulation is designed to ensure that the blow-off curve is straight and at a constant distance from the turbine's surge limit, enabling reliable operation even when the digital system breaks down. What can be considered a drawback to this method is that combining analog and digital signals requires complicated circuitry. Furthermore, the digital system in this case cannot assume the regulating process on its own when the analog regulator breaks down.